For classification purposes, we suggest that the general technical field of this disclosure be considered an optical element that varies a characteristic direction of a traversing optical light beam in proportion to a time-varying acoustical signal applied to a diffractive crystal, a so-called acousto-optic deflector (AOD). The diffraction of light that passes through the AOD is a function of time as the deflection-causing signal varies. The deflection-causing signal changes bulk optical characteristics of the diffractive crystal, varying diffraction through an optically transmitting medium. An ultrasonic wave generated at the surface or within the confines of the light control element sets up conditions in the element which produce a change in the optical parameters (e.g., refractive index) directly controlling the light.
Micronic Mydata AB (“Micronic”) builds capital equipment for mask making and direct writing to workpieces. The Prexision and LRS pattern generators make large area masks that are used, for instance, to create flat panel displays or televisions on large Generation 10 substrates in the latest pattern generator models. The Omega pattern generators make masks or reticles for chip making. Common to these pattern generators is use of scanned laser beams for forming latent images in resist on the surface of a workpiece, with scanning controlled by acousto-optic deflectors (AODs). This application describes an improved AOD that will have practical application beyond Micronic's pattern generators.
One AOD that Micronic has used is specified by the supplier to operate over a bandwidth of 100 MHz. In some applications, the driving frequencies applied to this AOD are centered around 200 MHz. In practice, it has been determined that this AOD can be used over a frequency range of around 130 MHz. Applying this bandwidth centered at 200 MHz, the lowest and highest frequencies in the operating range are 200−130/2=135 MHz and 200+130/2=265 MHz, respectively. Other AODs tested by Micronic have had a specified bandwidth of 150 MHz.
There is a general design rule of thumb that one must operate the AOD over less than one octave, i.e., the highest frequency must be lower than twice the lowest frequency. This rule of thumb avoids having the optics swept by the AOD collect higher diffraction orders (multiples of any frequency) of swept beams. To drive AODs, a frequency chirp of 5 MHz/μs has been used, generated by a direct digital synthesis (DDS) card. Consistent with the rule of thumb, this frequency chirp has a sweep time of about 24 μs. The so-called chirp produces a sawtooth pattern, which ramps up until it reaches the maximum frequency limit and then steps down to the minimum frequency limit. The chirp produces a repeating series of ramps and discontinuous steps down in frequency. In the following figures, only one cycle of the chirp is typically represented. Those of skill in the art will understand that the saw-tooth pattern continues indefinitely, through many cycles.
We disclose methods by which the old rule of thumb can be defied, resulting in an AOD with an extended frequency bandwidth, extended some 10-30 percent beyond one octave.
An opportunity arises to improve the sweep electronics of patterning and inspection systems. Better, faster, more efficient components and systems may result. The technology disclosed applies to both writing and reading with an optical beam.